Pickling solution and method



United States Patent 0 3,010,854 PICKLING SOLUTION AND METHOD Walter R.Satterfield, Baltimore, Md., assignor to Armco Steel Corporation, acorporation of Ohio No Drawing. Filed Dec. 31, 1954, Ser. No. 479,221

8 Claims. (Cl. 134-27) My invention relates to the treatment of metal,more particularly the brightening, pickling or de-scaling of metal andto the treating solution employed.

One of the objects of my invention is the provision of a simple,efiicient and reliable non-electrolytic method of de-scaling andbrightening metal such as stainless steel, the carbon steels, the lowalloy steels, titanium and its alloys, a1 'num 10 s, magnesium alloysand, in fact, all metals and alloys which acquire a scale or heat-tintin various processing and fabrication operatrons.

Another object is the provision of a method of the character indicatedwhereby heat-scale, heat-tint or other oxide film is removed from thesurfaceof metal, all without etch, pitting, intergranular or otherobjectionable attack of the surface of the metal itself, with a miningmpfmetaLremgr al, and without deposit of scum or the like duringtreatment and with the surface of the metal, following treatment g Icorrsr. 6 r to the atmosphere or to chemical forces and in conditionreceptive to the application of either electrolytic or nonelectrolyticcoatings of zinc, aluminum, copper, silver, nickel and the like, orother coatings such as baked and air-dryed paint, enamels and wrinklefinishing, ceramics and porcelains.

-A further object is the provision of a metal pickling, de-scaling orbrightening bath formed of readily available and comparativelyinexpensive materials which are easily and safely handled, whichrequires a minimum of attention and replacement and which is of longuseful life.

Other objects of my invention are obvious in part and in part pointedout in the description which follows.

My invention, therefore, resides in the combination of materials andcomposition of ingredients and the various operational steps and therelation of each of the same to one or more of the others as dwcribedherein and particularly set out in the claims at the end of thisspecification.

As conducive to a better understanding of certain features of myinvention it may be noted at this point that certain metals such asstainless steel, the titanium alloys and the aluminum alloys findapplication in a host of articles which are used under conditions ofoxidizing or corrosive atmosphere. And these metals are particularlysuited to such applications because of their inherent tendency to resistcorrosive attack, whether by acid base or salt and whether at roomtemperature or at elevated temperatures. It generally is held that thisproperty of corrosion resistance is had by virtue of a complex oxidefilm which forms on the surface of the metal itself.

Now in the fabricating and fashioning of the various corrosion resistingmetals into the host of articles of ultimate use there frequently isformed on the surface of the metal, as a result of some heatingoperation perhaps, an undesirable oxide film such as a heat-tint oroccasionally an even thicker film in the form of a scale. The formationof a heat-tinted surface and even an objectionable scale commonly isencountered in the conversion'of stainless steel ingots and billets intoplate, sheet, strip, bars, rods, wire, tubes and the like by hot workingmethods. The rolling, drawing or piercing temperatures employed commonlygive rise to scale forma- SRGS'S REFERENCE ice tion. And even in theheat treatment of the converted products or in the heat treatment of theultimate articles of use, as by age-hardening or precipitation-hardeningmethods, a light heat-tint occasionally is had.

The stainless steels affected in hot working operations or even in heattreating operations ordinarily contain 10% to 35% chromium either withor without nickel, the nickel content ranging up as high as 35%. In suchsteels other ingredients such as silicon, manganese, copper, aluminum,molybdenum, tungsten, columbiu-m, tit-anium, sulfur, phosphorus, carbonand nitrogen may be present in minor amounts. In several grades ofstainless steel the manganese content may amount to as much as 12%. Andin others the silicon content may be as high as 3 or 4%. The coppercontent, where copper is employed as an essential ingredient, may be asmuch as 5%. Similarly, the manganese and tungsten contents may amount toas much as 5% for the one and 2% for the other. The titanium andcolumbian contents seldom exceed about 1%. Sulfur and phosphoruscontents may reach .5% where these additions are employed for specialpurposes. So too the nitrogen content may amount to as much as .6%. Andthe carbon content may range anywhere from about .03% to .15%. It willbe understood, of course, that the remainder of the composition islI'OIl.

Because of the inherent nature of the stainless steels, and of thetitanium and aluminum al as well, any oxide film, such as a heat-tint ora scale, is very difficult to remove. Now in the prior art it frequentlyis necessary to resort to treatment in an alkali metal hydride bath asdescribed for example in the Gilbert US. Patent 2,448,262, or in a bathcomposed of alkali metal nitrates and hydroxides in accordance with themethod of the Webster and Falter US. Patent 2,458,661. And followingtreatment in these baths ofmolten salts the metal is immersed in a bathof commercial mineral acid, for example nitric hydrofluoric acid. Insome grades of stainless steel the metal is severely etched, thisproducing a dull, rough surface. many grades this 7 dr asticrtteatm entcauses intergpanplan corrosion. This is especially notica 1n thestanless steels of the chromium-nickel type wherein the carbon contentappreciably exceeds some .12%. And even in those grades which are notactually etched or pitted there often is found a dirty black smut whichis deposited out upon the surface of the steel. This smut generally issaid to be a deposit of metal carbides which are actually removed fromthe surface of the steel.

Ilhere, of course, is less ditficulty in removing the heattint or evenscale from the stainless steels of low alloy contents. Even with thosegrades, however, there is required a treatment in a pickling solution,as for example treatment in a nitric-hydrofluoric acid bath or treatmentin a bath comprising nitric acid with soluble fluoride salts and/orsoluble chloride salts. Here again, r, ce tain of these grades of steelare found to severely etch with the resultant formation oLadulLrouglrsurface. And certain of the austenitic chromium-nickel steels ofsubstantial carbon content are inclined to sufier through intergranularcorrosion.

It will be seen, therefore, that the de-scaling or pickling baths of theprior art are not entirely satisfactory for any substantial number ofgrades of stainless steel and that considerable care must be used toselect a particular bath and method of treatment for the particulargrade of steel to be treated. Such a practice is not entirelysatisfactory. I

One, of the objects of the present invention, therefore, is to provide abath which is adapted to remove the scale from a wide variety of gradesof stainless steel and from other metals and alloys as well, such as thecarbon steels,

EXAMINER the low alloy steels, the titanium alloys and the aluminum 5;alloys, and pickle and brighten these metals without surface etch,intergranular attack, or other appreciable metal remov'al. V'Referringnow more particularly to the practice of 5 my invention I find thatexcellent pickling and brightening of stainless steel and other alloysis achieved by treating these metals with an aqueous solution ofinorganic acid and hydrofiuosilicic acid or derivatives of the same.Almost equally good results are had where the metal is treated with an aue lution of inorganic acid together with hydrofiuoboric acid orderivatives. Ordinarily I prefer a treating solution essentiallyconsisting of one or more of the mineral acids together withhydrofiuosilicic acid or its derivatives. I generally prefer to employ atreating bath composed of one or more of phosphoric, glfuric p5 nitricacids, with or without acetic acid, together witha derivative ofhydrofiuosilicic acid such as one or more of the sodium, potassium,ammonium or magnesiumsilicoflnoricgs. I find, however, thatsubstantially "the same desired results are achieved with an aqueoussolution of mineral acid together with one or more of the methylamine,dibutylamine, ethylhexylamine, aniline and morpholine silicofluorides.As indicated above good results are had in a treating solution employingmineral acid together with one or more of the borofiuoridescorresponding to the various silicofiuorides. These various compoundsyield in solution a silicofluoride ion or a borofiuoride ion.

In general I find that excellent results are bad where the amount ofmineral acid ranges from about 2 to 75% by weight of solution. And theamount of compound yielding the silicofiuoride ion, or the borofluorideion as the case may be, amounts to about .5 to 40%. r A preferred bathcontains about 2 to 15% commercial nitric acid, 0 to 60% commercialphosphoric acid, .5 to commercial derivative of hydrofiuosilicic acidand remainder water. Actually, certain savings are had by substituting asmall amount of sulfuric acid for the nitric acid, but such a modifiedbath largely is limited in utility 40 to the products with a light scaleand where the acid conr centration is low. In such a bath phosphoricacid ordinarily is not employed. For the baths of the higher acidconcentration phosphoric acid preferably is included because of thebufiering efiect enjoyed. And I find that phosphate salts convenientlymay be employed as a substitute for phosphoric acid.

In accordance with the method of my invention the various metal productsto be treated are immersed in a bath of treating solution maintained ata temperature of 125 to about 195 F. At substantially lower temperaturesthe reaction is much too slow while-at substantially higher temperaturesthere is objectionable loss of active ingredient by evaporation. Ingeneral a temperature of around'170 F. is preferred, the bath beingconveniently heated by a steam line in order to assure the preferredoperating temperature. Immersion in such a bath for a period of one toforty-five minutes is suflicient to remove heat-tint or scale, the timedifiering largely with the nature of the metal being treated and theini- 50 tial condition of its surface oxidation.

With the steels of high total alloy content, as for example the straightchromium steels having a chromium content of 25% or more or theaustenitic chromiumnickel stainless steels having a total chromium andnickel content amounting to 25 or more, 1 preferably subject the same toa scale softening treatment in advance of actual de-scaling. Especiallyis this beneficial in the case of converted products such as plate,sheet, strip, bars, rods, wire and the like in which the conversion ishad by hot-working or hot-cold-working methods.

In accordance with the practice of my invention the converted productsare first given a scale softening or scale loosening treatment,conveniently referred to as a scale conditioning treatment, by immersionin a molten 7 bath of sodium hydride or a bath of molten sodiumhydroxide and sodium nitrate. The products then are withdrawn from thebath and washed either in a tank of water containing a small amount ofacid or washed with a hose. Following this I immerse the products in abath of nitric acid with or without phosphoric acid and together with aproduct such as a derivative of hydrofluosilicic acid yielding thesilicofluoride ion. Equally good results are had where the convertedproducts are first treated in a bath of molten sodium hydroxide andsodium nitrate followed by water rinse and immersion in an aqueous bathof the nitric acid with or without the phosphoric acid, but containingthe hydrofiuoboric acid derivative.

Certain savings in the cost of scale removal are achieved in wire andother converted products by subjecting them to a mechanical flexing overrolls to loosen or condition the scale, and then immersing the productsin the acid bath containing the derivative of the hydrofluosilicic acidor the hydrofiuoboric acid.

While precipitation hardened fabricated articles fashioned of stainlesssteel generally are free of scale they occasionally have a lightlyheat-tinted surface. Ordinarily the heat-tinted articles are cleaned andbrightened by immersion immediately in my bath of mineral acid andhydrofluosilicic acid derivative (or hydrofiuoboric acid derivative)although I occasionally find benefit in first subjecting the articles toan aqueous bath of 10% sodium hydroxide containing potassiumpermanganate in small amount followed by treatment in my scale removingbath. Benefit also occasionally is bad by preliminary treatment inaqueous sulfuric acid with or without one or more chlorides. Suchtreatment serves to soften the scale and condition it for easy removalby immersion in the mineral acid bath containing the hydrofiuosilicicacid derivative (or hydrofiuoboric acid derivative).

I find that stainless steel converted products treated in accordancewith my method, and various fabricated stainless steel products sotreated as well, have a better surface color with less surfaceroughening than products and articles treated with the prior artprocesses. Moreover, I find that my products and articles are singularlyfree from intergranular attack and that there is minimum of metalremoval. Furthermore the products and articles leave the bath in acondition which is less susceptible to atmospheric corrosion, that isrusting or other staining or streaking. And the converted products arein better surface condition for further working or forming, such as deepdrawing, drawing into wire and the like, particularly where the scaleremoval treatment is had in a bath including the phosphoric acid. This Iwould attribute to the presence of a phosphate coating on the surface ofthe steel. The various stainless steel products and articles pickled,cleaned or brightened in accordance with my invention are particularlyfree of any surface pitting, whether the bath has just been newly madeor has been in use for some time. This is somewhat surprising because inmany of the treating methods of the prior art an old bath is found topit the metal, this apparently because of the high iron content of thebath. I attribute the freedom from pitting had in the method of myinvention to the formation of complex ions, for example ironsilicofiuorides, which do not have the same adverse effect upon thesurface of the metal as the ions found in the used baths of the priorart.

There is a further advantage of the bath of my invention in thatevaporation losses are minimized as compared to the known commercialbaths employing hydrofluoric acid as a necessary ingredient. Moreover,the ingredients of my bath are much easier to handle than those of theprior art in'that the hydrofiuosilicic acid derivative is in the form ofa dry crystal powder as distinguished from the liquid hydrofluoric acidof the prior As specifically illustrative of the practice of myinvention I give below three examples of treatment in which excellentresults are had:

(1) Armco 21-4 N stainless steel (21% chromium, 4% nickel, 9% manganese,.40% nitrogen and remainder iron) exhaust valve steel hot rolled andannealed rod material in A6" and /s" sizes was subjected to immersion inmolten sodium hydride bath for about ten minutes followed by waterquenching. The rod was then pickled for seven and a half minutes in abath consisting of 19% commercial orthophosphoric acid, 7 /2% commercialnitric acid, 4% magnesium silicofluoride and the remainder water, allpercentage figures being by weight, and the bath being held at atemperature of about 170 F. After rinsing in tap water the samples ofrod were found to have an excellent clean surface, free of pits and withno undue darkening or etching. Metallographic examination revealed therod to be without traces of intergranular corrosion.

(2) Further samples of Armco 21-4 N stainless steel exhaust valve rod inthe hot rolled and annealed condition were treated in a sodium hydridebath for ten minutes followed by water quenching after which they arepickled for seven and a half minutes at 170 F. in a bath consisting of5% commercial nitric acid, 2% commercial magnesium silicotluoride andthe remainder water, all by weight. An excellent clean bright surfacewas had which was free from scale and free from undesirable etching.Here again, metallographic examination revealed the surface to be freeof intergranular corrosion.

(3) Sample sheets, .060 inch thick, of the Armco 17-7 pH precipitationhardening stainless steel (17% chromium, 7% nickel, 1% aluminum andremainder iron) in the commercially annealed condition were given a1050" F. precipitation hardening treatment. Close examination showedthat the hardened steel samples had a tight thin grey blue scale. Thiswas removed by pickling for five minutes in a bath consisting of 10%commercial phosphoric acid, 7% commercial nitric acid, 4% commercialsodium silicofluoride and remainder water, the bath being maintained ata temperature of about 175 F. A good clean bright surface was producedwhich showed no evidence of etching. Here it will be noted that no scaleconditioning treatment was given and yet excellent results wereachieved.

While, as more particularly indicated above, many grades of stainlesssteel are pickled and brightened with excellent results with my aqueousbath of nitric acid, with or without phosphoric acid, but containing thehydrofluosilicic acid derivative (or derivative of hydrofluoboric acid)I find that all commercial grades of stainless steel are satisfactorilyde-scaled, pickled and brightened with my process wherein there first ishad a scale conditioning treatment as by scale softening or scalecracking (mechanical flexing over rolls) followed by the treatment inthe nitric acid bath, with or without phosphoric acid, and acidderivative. Moreover, the method of my invention satisfactorily removesscale, pickles and brightens other alloy steels such as the siliconsteels and the tool steels and certain non-ferrous metals and alloyswhich are found with extremely refractory oxide coating such as titaniumand titanium alloys as well as the magnesium alloys and aluminum s.

Thnmv il fiae seen that I have provided in my invention a method and apickling bath in which the various objects hereinbefore noted togetherwith many practical advantages are successfully achieved. The method issimple, reliable and does not require the necessity for expensiveelectrical equipment. And the bath is easily prepared and achieves longuseful life with a minimum of replacement.

As many possible embodiments may be made of my invention and as manychanges may be made in the embodiments hereinbefore set forth it will beunderstood that all matter herein is to be interpreted as illustrativeand not as a limitation.

I claim as my invention:

1. In the brightening, pickling and de-scaling of steel, the art whichcomprises conditioning any scale on said steel for subsequent removal;and then subjecting the steel to an aqueous t Mon comprising 2% to 75%of at least one of the group consisting of phosphoric, sulfuric andnitric acids, and .5 to 40% of at least one of the group consisting ofsodium, potassium, ammonium, and magnesium silicofluorides.

2. In the brightening, pickling, and tie-scaling of steel, the art whichcomprises subjecting the steel to an aqueous treating solutioncomprising 2% to 15% of at least one of the group consisting ofphosphoric, sulfuric and nitric acids, together with .5 to 40% of atleast one of the group consisting of sodium, potassium, ammonium andmagnesium silicofluorides.

3. In the brightening, pickling and de-scaling of stainless steel ofhigh total alloy content, the art which comprises subjecting the steelto an aqueous treating bath essentially comprising by weight about 7%nitric acid, about 10% phosphoric acid, and about 4% magnesiumsilicofiuoride.

4. In the de-scaling of stainless steel of 25% or more total alloycontent, the art which comprises conditioning the metal scale forsubsequent removal by immersing the steel in a bath of molten sodiumhydride; removing from the bath and washing in water; and thensubjecting the steel to an aqueous solution essentially comprising 2% to15 of at least one inorganic acid and .5% to 40% of at least onecompound of the group consisting of derivatives of hydrofluosilicic acidyielding a silico fluoride ion and derivates of hydrofluoboric acidyielding a borofiuoride ion.

5. A solution for brightening, pickling and de-scaling steel comprisingabout 7% nitric acid, about 10% phosphoric acid, about 4% magnesiumsilicofiuoride, and remainder water.

6. An aqueous solution for brightening, pickling and de-scaling steelcomprising by weight about 2% to 15% nitric acid, up to about 60%phosphoric acid, and about .5 to 40% magnesium silicofluoride.

7. In the de-scaling of steel, the art which comprises conditioning themetal scale for subsequent removal by immersing the steel in a bath ofone of the group consisting of aqueous sodium hydroxide with potassiumpermanganate, aqueous sulfuric acid, and aqueous sulfuric acid withchlorides; and subjecting the steel to an aqueous solution comprising 2%to 75% of at least one of the group consisting of phosphoric, sulfuricand nitric acids,

and .5% to 40% of at least one of the group consisting of sodium,potassium, ammonium and magnesium silicofluorides.

8. In the de-scaling of steel, the art which comprises conditioning themetal scale for subsequent removal by mechanically flexing the steelover rolls; and subjecting the steel to an aqueous treating solutioncomprising 2% to 75% of at least one of the group consisting ofphosphoric, sulfuric and nitric acids, and .5 to 40% of at least one ofthe group consisting of sodium, potassium, ammonium and magnesiumsilicofluorides.

References Cited in the file of this patent UNITED STATES PATENTS921,350 Bishop May 11, 1909 2,337,062 Page Dec. 21, 1943 2,340,328 JonesFeb. 1, 1944 2,383,800 Johnson Aug. 28, 1945 2,413,365 McCoy Dec. 31,1946 2,422,066 Bacon June 10, 1947 2,477,181 Holman July 26, 19492,542,779 Neill Feb. 20, 1951 2,655,481 Clingan Oct. 13, 1953 2,711,364Beach June 21, 1955 2,719,079 Murphy Sept. 27, 1955

4. IN THE DE-SCALING OF STAINLESS STEEL OF 2K% OR MORE TOTAL ALLOYCONTENT, THE ART WHICH COMPRISES CONDITIONING THE METAL SCALE FORSUBSEQUENT REMOVAL BY IMMERSING THE STEEL IN A BATH OF MOLTEN SODIUMHYDRIDE, REMOVING FROM THE BATH AND WASHING IN WATER, AND THENSUBJECTING THE STEEL TO AN AQUEOUS SOLUTION ESSENTIALLY COMPRISING 2% TO15% OF AT LEAST ONE INORGANIC ACID AND .5% TO 40% OF AT LEAST ONECOMPOUND OF THE GROUP CONSISTING OF ION AND DERIVATES OF HYDROFLUOBORICACID YIELDING A BOROFLUORIDE ION.
 6. AN AQUEOUS SOLUTION FORBRIGHTENING, PICKLING AND DE-SCALING STEEL COMPRISING BY WEIGHT ABOUT 2%TO 15% NITRIC ACID, UP TO ABOUT 60% PHOSPHORIC ACID, AND ABOUT .5% TO40% MAGNESIUM SILICOFLUORIDE.